Email templates have a new Relationship section in the merge-field picker:
{{relationship.first_name}}{{relationship.last_name}}{{relationship.full_name}}{{relationship.email}}{{relationship.phone}}
They fill in details of the contact this person is linked to, for example a spouse kept as a separate contact. A template can now say “Hi Mark, it was great meeting you and Elena” without anyone typing the name.
If a contact has several relationships, the first one added is used. If a contact has no relationship, the fields are left blank. When an automation emails a related contact, these fields show the contact receiving the email.